1. Field of the Invention
The present invention relates to the field of absorbent granules produced from naturally occurring sorptive minerals, and in the past have consisted almost entirely of absorbent clay granules. The granules have various industrial and household uses such as the absorption of oil and grease and the like on floors and elsewhere where these materials represent a safety hazard, they are used in an animal toilet application as a cat box absorbent, they are used as a carrier for chemicals such as pesticides, herbicides and the like in various agricultural and horticultural applications, and they are used as ground cover in their natural color. The invention comprises a method for utilizing hydrated gypsum plaster. Gypsum as a naturally occurring mineral, or the powdered plaster therefrom, are not suitable as a moisture absorbent material such as is absorbent clay, but by controlled processing absorbent granules can be made with properties substantially equivalent to those of the absorbent clay granules.
More particularly, the present invention relates to a process utilizing standard commercially available plaster, or plaster of Paris as such plaster is often called, in powdered form made from gypsum. The plaster is mixed with water, or water and other materials, according to predetermined specifications for a mix. Variations in the specifications for the synthetic absorbent produce variations in the characteristics of the final absorbent granules produced thereby with respect to bulk density, moisture absorption, hardness, and stability in water or other liquid.
The absorbent granule product of this invention is consistent from mix to mix with respect to the specifications provided for the process, depending upon the plaster being the same from mix to mix, for it is known that plaster can vary in constituency. The characteristics of a plaster can be determined in advance of starting a production run so that adjustments can be made to compensate for undesired variations. Such consistency of product cannot be obtained in the manufacture of absorbent granules from the naturally occurring sorptive minerals such as the absorbent clays.
2. Description of the Prior Art
Beginning in the late 1930's and early 1940's, naturally occurring sorptive minerals, such as the clay which is known as fuller's earth, and having a natural porous structure, were first used as absorbents for oils, greases, water and other undesirable substances on the floors of factories and shops, filling stations, and the like. Prior to that, the major use of fuller's earth had been in the refining of oils. Because of the low bulk density of such material, in the range of 25 to 55 pounds per cubic foot, and because of its porosity and its absorbent properties, other uses were developed. Granules of fuller's earth found a large market as a use in pet animal toilet boxes, particularly cat boxes, and as a carrier for chemicals for agricultural purposes, as mentioned above. This density range represents what is considered to be a light weight material.
More specifically, as to the naturally occurring absorbent or sorptive mineral clays, these are mined, dried or calcined, crushed, and screened to a size determined by the particular use for which the material is intended.
For example, the usual size is 6.times.60 screen mesh for oil and grease absorbent, and for cat box absorbent. However, granules for each purpose can be of many different sizes, and 10.times.60 mesh, 6.times.16 mesh, and 6.times.24 mesh sizes have been marketed. A special size such as a very coarse 4.times.10 mesh has been used for either one of the above two purposes. For oil and grease absorbents, it is also desirable that the granules be resistant to crushing and to sticking to the floor when walked on, or travelled over by plant equipment such as lift trucks.
With respect to screen mesh sizes, the sizes for the two above discussed granules can vary quite widely, and larger size granules which are approximately 1 to 2 inches in diameter or in the cross dimension have been used for decorative ground cover.
For sorptive mineral clay granules when used as carriers for insecticides, or pesticides, or herbicides, the specifications are tight. Sizes which are commonly specified are 8.times.16, 16.times.30, 30.times.60, and 24.times.48, and each size has specified tolerances such as screen size, and bulk density. A representative specification for one such product is that the bulk density must be measured by the well known Ohaus Loose Bulk Density method, and such bulk density can be 31 to 35 lbs. per cu. ft. The screen size tolerance is that a maximum of 2% by weight of the granules can be larger than 20 mesh, and none larger than 10 mesh. There must not be any more than 11/2% to 2% smaller by weight than 60 mesh for the granules. These mesh sizes are Tyler standards. In addition, it has been specified that there must be between 5 and 8 million particles or granules per pound of absorbent product.
For such chemical carriers, lack of dust in the product is important. It is also important that the specified properties of the absorbent clay granules be consistent from shipment to shipment since the distribution of the different chemicals by means of the granules is a carefully controlled operation.
From the above specific characteristics which are generally required, it is clear that a substantial degree of control should be maintained in processing the naturally occurring minerals such as absorbent clays in order to satisfy each of the numerous uses discussed for the granules. However, such control is not possible except by careful selection of the clay to be processed. Size can be varied by crushing and sizing, and the hardness and stability in water of the granules can be varied by calcining the sorptive mineral, but that is substantially the limit in the controlled variation of properties or characteristics of the absorbent granules made from naturally occurring minerals such as clay. There is no practical way to change the bulk density or the absorption capacity of such clays.
The lack of uniformity in bulk density in the clays at various mining and processing operations creates a problem in packaging. If the bulk density is high relative to the norm for a particular package size of granules, a predetermined amount such as 5 or 50 pounds will only partially fill a package or bag of a predetermined size. If the bulk density is too low or the clay is very light in weight relative to a norm, the package or bag will not accommodate the required weight of granules.
The usefulness and effectiveness of the absorbent clay granules for the purposes discussed above also vary with respect to the different mines in different locations from which the clay is obtained.
In addition to the preceding problems, absorbent clay as the principal sorptive mineral suitable for the purposes described is found only in limited areas in the United States and in some other countries of the world, and is not found at all in most industrial areas in the world. As a result, the cost of transportation overall from the sources of such clay to all the markets of the world is a substantial part of the total selling price. In fact, for the United States and perhaps even more so in Europe, the absorbent clay with the desired characteristics and suitable for the purposes described is found in such limited areas that a substantial percentage of the delivered cost of this bulky product is in its transportation cost.
With ever expanding use for the absorbent granules such as clay granules, and with the limited areas where such clay has been available, extensive searches for new sources of such raw materials have been undertaken by the industry. However, no substantial new deposits of sorptive minerals have been found which serve the same purposes as the absorbent clays, and no equivalent naturally occurring absorbent materials have been found which can be economically produced and marketed. Consideration has been given to synthetic substitutes, but until the product made by the process of the present invention was discovered, none has been found which has acceptable properties and characteristics, is competitive in cost, and for which the sources of supply are so located as to minimize the cost of transportation.
Considering that the need for such absorbents is world-wide, and the greatest need is in industrial, agricultural and urban areas, most of which are far removed from the sources of sorptive minerals such as absorbent clay, it is clear that the overall problems with this naturally occurring product have created hurdles for some time that the industry must surmount.